In the prior art cascaded diffractive elements have been used to increase diffractive efficiency. In addition processing of the beam between diffractive elements is also used. However, processing has been done in the far field. Processing in the far field, where the beam is collimated, and is a Fourier transform of the original beam, makes optical processing of individual or selected wavelengths or channels very complex and difficult, requiring computer processing holographically. Generally the prior art far field processing is used for global processing, to all wavelengths, since local processing of individual wavelength bands is difficult.
An application of optical filtering in the far field of a dispersed beam is disclosed in U.S. Pat. No. 5,805,759 issued to Fujitsu Limited, Sep. 9, 1998. This application discloses an optical equalizer comprising an attenuating filter between two dispersion gratings. Light is processed as a collimated beam. The attenuator introduces variable attenuation primarily to compensate for gain tilt from amplifiers. One hundred percent attenuation can be selectively arranged using metalized blocking strips. Because filtering is performed on the collimated beam, obtaining sufficient wavelength separation is more difficult. It requires high dispersion and long distances between the gratings. The size requirements makes the proposed device impractical for use. Further, working at high dispersion causes greater sensitivity to polarization. Filtering as taught by the Fujitsu application only involves attenuation at varying levels within a dispersed beam. Displacement of the dispersed wavelength bands is not contemplated. The Fujitsu patent does not teach more complex processing achieved by displacing wavelength bands in the focal plane, which can be achieved in a compact device by processing focused wavelength bands in the focal plane.
The present invention has found that optical processing, particularly of selected wavelengths, is more easily accomplished in the focal plane where the wavelengths are more easily separated within a compact space and can be physically processed or moved without complex calculation. In the focal plane position can be correlated to wavelength. This can be used to provide filtering for complex systems. It is often desired to redirect selected wavelengths for add/drop application, or to shift or invert wavelength bands for flat top or other specialized response profiles not possible through simple attenuation.
Simple focal plane optical processing is disclosed in U.S. Pat. No. 5,223,405 issued in 1993 to Hewlett-Packard Company. This reference discloses the use of a slit to permit a selected portion or wavelength band of a dispersed light beam from a diffraction grating to pass. The selected portion of the light beam is then redirected to the diffraction grating to be recombined in order to pass to an output for spectral analysis. The isolation of a selected wavelength band is important for wavelength monitoring and spectral analysis. However, the method and apparatus taught by the Hewlett-Packard reference does not suggest the controlled filtering of multiple discrete wavelengths and varying intensities or multiple inputs which can be achieved by processing the dispersed light in the focal plane.